1. Field of the Invention
The invention concerns novel optical security articles which can be used as transparent overlays to protect documents from tampering. The invention also concerns methods for making such articles.
2. Related Art
Documents often have adherent transparent overlays to provide protection against dirt, moisture, and general wear and tear. A typical protective transparent overlay has a plastic film bearing an aggressive adhesive layer by which it can be permanently adhered to the face of a document. Transparent overlays are known which do not obscure underlying information, such as those having legend-containing sheet materials which have retroreflective legend and retroreflective background areas that are substantially indistinguishable under ordinary diffuse light viewing conditions but are clearly distinguishable under retroreflective viewing conditions. Attempts to tamper with information over which the overlay has been applied result in readily detectable disruption of the overlay.
Other transparent overlays can be imaged with a pattern or legend, such as a holographic pattern or image that is readily noticeable only when viewed retroreflectively, and can be adhesively bonded to a document without obscuring the face of the document. Both reflecting and transparent hologram overlays are known. Holographic overlays, however, suffer from the problem that the hologram surface relief pattern is typically produced using a pattern of light interference fringes formed by the intersection of two coherent light beams, typically laser beams, which requires expensive lasers and other equipment. It would be advantageous if a transparent overlay could be developed which would avoid the use of holograms.
Shaped plastic articles, including substantially totally internal reflective films, have been made by replicating a microstructure-bearing surface with an ultraviolet radiation-curable organic oligomeric composition; however, their use as protective and/or security overlays for important documents and other articles is not known or suggested.